The present invention relates to a method and an apparatus for separating particles from hot gases.
Thus, the invention relates to a centrifugal separator assembly and a method of separating particles in a centrifugal separator assembly attached to a fluidized bed reactor, for separating solid particles from gas exhausted from the reaction chamber of the fluidized bed reactor, which separator assembly comprises a vortex chamber, which is in a horizontal direction, defined by vertically-extending outer walls formed of planar water tube panels, the inside of the walls being provided with a refractory lining and defining a gas space in the vortex chamber, in which at least one vertical gas vortex is established, at least one inlet for introducing gas into the gas space from the reaction chamber, at least one outlet for discharging purified gas from the gas space, and at least one outlet for discharging separated solid particles from the gas space.
The present invention relates especially to centrifugal separators utilized for separating solid particles from the process and product gases of fluidized bed reactors, especially, circulating fluidized bed reactors used for combustion or gasification of carbonaceous or other fuels.
It is generally known how the inlet and outlet ducts of a centrifugal separator should be arranged so as to make the flue gas entering through the inlet duct produce a vertical gas vortex. Conventional centrifugal separator assemblies include one or more centrifugal separators, i.e., cyclones, defined by an outer wall having a shape of a right circular cylinder, and a conical bottom. The cyclones of a fluidized bed reactor are traditionally manufactured as uncooled structures provided with a refractory lining, though the walls of the fluidized bed reactor itself are generally formed of cooled water tube panels. When connecting an uncooled particle separator to a cooled reaction chamber, it is necessary to consider varying thermal motion and use such arrangements that enable relative motion, even if the arrangements are expensive and susceptible to damage. Cylindrical cyclones have also been manufactured as structures formed of wafer tubes, whereby the temperature difference between the cyclone and the cooled reactor chamber remains small. However, to provide a water tube wall construction of a cylindrical form and to connect it to the surrounding constructions requires a lot of manual labor and is, therefore, expensive.
U.S. Pat. No. 4,880,450, for example, discloses a method by which a cooled cylindrical cyclone can be connected to the furnace of a fluidized bed boiler and to the heat recovery section thereof. The cylindrical upper section of the cyclone comprises water or steam tubes attached to each other, the inner surface of which is covered with insulative material. The separator according to this patent can be connected to a cooled environment without separate elements enabling relative motion, but the construction requires a lot of effort and is, therefore, expensive.
U.S. Pat. No. 5,281,398 discloses an arrangement in which particles are separated from hot gases in a centrifugal separator, the vortex chamber of which is composed of planar water tube panels. Thus, the gas space of the vortex chamber is polygonal in horizontal cross section, preferably quadrate or rectangular. This kind of separator is inexpensive to manufacture and can easily be connected to a reactor furnace formed of similar wall panels, whereby a compact unit is established. Traditionally, the gas volume of a separator vortex chamber is cylindrical, as the cylindrical space interferes with maintenance of the gas vortex velocity to as small a degree as possible. The invention disclosed in U.S. Pat. No. 5,281,398 is, however, based on the fact that a gas vortex can also be established in a space polygonal in cross section. In a cylindrical separator, the particles separated by centrifugal forces are driven to the vortex circumference and flow downwardly along the inner walls of the vortex chamber. Appropriate operation of a polygonal separator is based on the fact that the corners of the gas space enhance the separation of the particles and serve as suitable flow-down areas for the separated particles.
U.S. Pat. No. 4,615,715 discloses an assembly in which a cylindrical cyclone manufactured of abrasion-resistant material is disposed inside a cooled enclosure which is quadrate in cross section. In this arrangement, the shape of the gas space is ideal for maintaining the vortex velocity. Nevertheless, the manufacture of the water tube panels for the separator enclosure can be automated, and the separator can straightforwardly be connected to a cooled environment. In the arrangement according to this patent, the relatively large space between the annular inner space and the quadrate outer enclosure is filled with suitable material. The problem with this material is that it serves as a heat insulator and increases the weight and heat capacity of the separator. Thus, it increases the temperature of the separator""s inner wall during operation and adds to its thermal inertia. Large and rapid changes of temperature can cause damage to the material in the intermediate space, which adds to the maintenance and repair costs. Therefore, the changes of temperature in the separator need to be sufficiently slow, which fact is to be considered when changing the capacity of a plant and especially during start-ups and shut-downs. Further, the innermost surface of the material has to be very abrasion-resistant and, therefore, the filling of the intermediate space is done by a special multi-layer technique. This however, adds to the construction costs and makes the separator structure complicated.
It is an object of the present invention to provide an improved centrifugal separator assembly and a method of separating particles from hot gases.
In particular, it is an object of the present invention to provide a compact centrifugal separator assembly and a method of separating particles, which assembly is less expensive to manufacture and the degree of particle separation of which method is high.
Moreover, it is an object of the present invention to provide a method of separating particles and a centrifugal separator apparatus with minor need for maintenance, which apparatus can, preferably, be connected to a cooled reaction chamber.
In order to achieve these and other objects, a centrifugal separator assembly, as set forth in the claims, is provided.
Thus, it is characteristic of the centrifugal separator assembly according to the present invention that the vertically-extending outer walls of the vortex chamber form at least one corner, the angle between the sides of which exceeds ninety degrees, the corner being rounded by a refractory lining on the inside of the outer walls.
In order to achieve the objects, a method of separating particles, as set forth in the claims, is also provided.
Thus, in one aspect according to the present invention, the gas exhausted from the reaction chamber of a fluidized bed reactor is, in the vortex chamber, brought to hit at least one corner rounded by a refractory lining on the inside of the outer walls, the angle between the vertically-extending outer walls of which corner exceeds ninety degrees.
The arrangement according to the present invention combines the advantages of planar cooling surfaces and a rounded gas space and avoids the disadvantages of thick refractory layers by providing the outer wall of the vortex chamber with a polygonal horizontal cross section, in which at least some of the angles are more than ninety degrees.
Separators according to U.S. Pat. No. 5,281,398, in which the gas space of the vortex chamber is a polygon in horizontal cross section, operate flawlessly in normal operating conditions. It has been discovered, however, that a particularly advantageous construction can be provided for new generation gas separators by using gas velocities and separator design standards that differ from those used earlier. As such development of separators is further encouraged, the angles of the gas space may, in some applications, cause restrictions for the total design of the reactor.
It has been discovered that in some applications, the operation of a polygonal separator can be further improved by rounding off one or more corners formed by the outer walls of the vortex chamber. Further, so as to minimize structural problems and problems related to the durability of the construction caused by rounding of the corners, it is essential in the present arrangement that the angle between the planar panels of the outer wall of the vortex chamber is, at the rounded outer corner, distinctly over ninety degrees.
It has previously been known on the basis of U.S. Pat. No. 5,738,712 that the gas flow entering a rectangular vortex chamber and the gas vortex in the vortex chamber can disrupt each other unless the gas vortex is redirected in the direction of the incoming jet in the corner formed by a partition wall connected to the inlet opening. The present invention is, however, related to another problem, i.e., a problem of the gas vortex possibly remaining less optimal in the corner area of the vortex chamber.
When a vertical circular cylinder is surrounded by four vertically-extending planar panels perpendicular to each other and in a tangential relationship to the cylinder, the distance between the planar panels and the cylinder surface at the corners is about 0.414 times the cylinder radius. Consequently, if refractory lining is provided so that the thickness of the layer in the middle of the planar panels is, e.g., 0.05 times the cylinder radius, the layer would be more than eight times thicker at the corners. Thus, especially in the corner areas, the thermal conductivity of the refractory layer may be low, and the cooling of the outer surface is not necessarily able to keep the temperature of the inner surface low enough. Moreover, the varying thickness of the refractory lining can cause considerable temperature differences and thereby increase the risk of the layer getting damaged. A thick layer also adds to the weight of the structure and thereby causes problems related to supporting the structure.
If the cylinder is surrounded by five panels, instead of four, the angle between the panels is 108 degrees and the distance between the panels and the cylinder surface is only 0.236 times the cylinder radius at the corners. With six, seven, and eight panels, the angles therebetween being 120, 128.6, and 135 degrees, respectively, the distance is 0.154, 0.110, and 0.082 times the cylinder radius, respectively. Thus, the maximum thickness of the refractory layer as well as its weight and heat capacity decrease substantially, even when the angle of the separator corner is, e.g., 108 degrees instead of 90 degrees. If the angle is 135 degrees, the maximum layer thickness required by the rounding is only a fifth of what is required by the rounding of a right angle. The thermal conductivity of a thin refractory lining is high and relatively even in the various parts of the outer wall of the vortex chamber, whereby the maximum temperatures of the layer under operation decrease and the temperature differences in the various wall parts are diminished.
According to a preferred embodiment of the present invention, each separator corner is rounded and approximately of the same size. In this case, the number of corners is preferably five, six, seven, or eight and the angles are preferably about 108, 120, 128.6, or 135 degrees, respectively. When the number of separator corners is six or eight, a plurality of separators can preferably be connected to each other and/or to the furnace. Most preferably, the separator has eight corners, whereby the parallel walls between the separator and the reaction chamber as well as between adjacent separators can be utilized when designing the structure. However, in some special cases, i.e., for arranging a particular support structure and a gas inlet duct, it can be advantageous also to manufacture separators in which the number of corners is odd.
According to another preferred embodiment, only some particle separator corners are rounded. In this case, the sizes of the rounded corners can be different from the ones mentioned above. Preferably, though, the angles are between about 110 to about 150 degrees, and more preferably about 135 degrees. Most preferably, a separator including angles of various sizes can have a basic shape of a polygon, some angles being right angles and not rounded and the other angles being beveled by a planar panel and rounded by a refractory lining.
According to one preferable arrangement, a particle-laden gas flow entering through an inlet opening hits first, nearly perpendicularly, a wall or the other side of a right-angled corner, but after the first impact, the gas flow hits at least one rounded corner. In this kind of arrangement, the first corner or wall in the vortex chamber serves as a suitable spot for separating particles, but in the rounded corners after that, the aim is to maintain the velocity of the gas flow at as high a level as possible.
The rounding of the corners can preferably be arranged so that in the section of the vortex chamber outer wall that includes a plurality of corners, the vortex chamber inner wall is continuously cylindrical. In other words, the radius of curvature of the rounding is approximately the same as the distance between the center of the vortex established in the vortex chamber and the inner wall of the vortex chamber. Another preferable way is to provide separate rounding in each corner area, whereby the radius of curvature of the rounding is smaller than mentioned above, and a straight inner wall surface remains between the rounded parts requiring only a thin, even refractory lining to protect the wall. The thickness required by the even refractory lining depends on the materials used and the operational conditions, being typically at least about 15 to about 70 mm. In order to achieve the benefits gained by the rounding according to the present invention, the radius of curvature should not be too small. Preferably, the radius of curvature of the rounding is at least about one third of the radius of the vortex established in the vortex chamber, i.e., of the distance between the vortex center and the inner wall of the vortex chamber.
When using a short radius, curvature of the roundness of the chamber is not complete, but the amount of the refractory lining on the walls is even smaller than in the case of a continuously cylindrical vortex chamber. In some cases, due to the varying characteristics of the corners, it can be preferable to use various radii of curvature for the rounding in different corners. A special case according to this principle is the one in which one or more corners formed by the outer walls are rounded and one or more corners are not rounded.
The horizontal cross section of the vortex chamber can preferably be either nearly circular, whereby only one gas vortex is established in the vortex chamber, or oblong and shaped in a manner allowing more than one gas vortex to be established in the vortex chamber, i.e., the dimension of the vortex chamber extending in the direction that the reaction chamber wall closest to the vortex chamber extends is preferably about twice the depth perpendicular to the width, whereby two adjacent gas vortices can preferably be established in the vortex chamber.
The gas inlet ducts to a vortex chamber of two gas vortices are located most preferably in the middle of the vortex chamber wall on the reaction chamber side, but they can also be disposed separately from each other, in the proximity of the outer corners of the vortex chamber wall on the reaction chamber side. The wall facing the inlet ducts arranged in the middle of the wall in the vortex chamber of two vortices on the reaction chamber side can be straight, whereby the gas flow entering the vortex chamber side can be straight, whereby the gas flow entering the vortex chamber hits the wall almost perpendicularly. Alternatively, a wall section formed of planar water tube panels and that is triangular in cross section can be provided in the middle of the wall, by rounding of which wall section the gas flow is brought to hit a rounded wall first.
To ensure structural strength and high separation capacity, two or more smaller separators, instead of one large separator, are often constructed in a large reaction chamber. When using several cooled cylindrical separators, the large proportion of manual work adds to the cost excessively. Thus, for economical reasons, it is sometimes necessary to use larger separators than are, in fact, optimal. In these cases, it is not always certain that a high separation capacity can be accomplished in all conditions, and therefore, to ensure the structural strength, space consuming and cost increasing arrangements have to be used. When using the structure according to the present invention, even small separators can be manufactured at low cost, whereby such separators, being easy to support and optimal as regards the separation capacity, can be used.
When the outer walls of the vortex chambers manufactured according to the present invention include, e.g., eight angles, two adjacent vortex chambers can preferably be arranged so that their sides run parallel, whereby the parallel wall panels of the vortex chambers can straightforwardly be connected to each other. The adjacent vortex chambers can also advantageously be interconnected in such a manner that they share a common straight wall section.
Centrifugal separator assemblies according to the present invention can preferably be arranged in conjunction with a reaction chamber so that some of the planar outer wall panels of the vortex chamber are parallel to the planar wall of the reaction chamber, whereby the vortex chamber can easily be attached to the reaction chamber wall. The vortex chambers can also advantageously be manufactured so that the wall sections of the vortex chambers on the reaction chamber side are shared by the reaction chamber.
The possibility of using common wall sections between two separators or between the separator and the reaction chamber is one of the advantages of a separator formed of planar tube panel walls, as it makes it possible to reduce the manufacturing costs considerably. The common wall sections cannot, however, be easily supported from either side of the wall section, whereby the width of this kind of common wall has, in practice, a certain maximum limit. If the limit is exceeded, two separate walls have to be used. Thus, the support arrangements for the common wall sections can in some cases prevent the utilization of large separators of optimum size.
The width of the planar outer wall of a rectangular separator is always at least as large as the vortex diameter, but the width of an individual wall in the separator according to the present invention can be distinctly smaller than the vortex diameter. Thus, one of the advantages of the separator according to the present invention is that the aforementioned problem related to supporting the common wall sections is encountered only with separators in which the gas spaces have diameters larger than the gas spaces in rectangular separators that encounter the problem.
On the basis of the above, the diameter of the vortex chamber in a particle separator according to the present invention can, in each individual case, be optimized more freely than the diameter of the vortex chamber in cooled cylindrical particle separators or separators with a rectangular outer wall can be optimized. Preferably, the diameter of the vortex chamber according to the present invention is about 3 to about 8 m, e.g., about 5 m.
Since the vortex chambers according to the present invention are not rectangular in cross section, free triangular spaces are established when the vortex chambers are connected to each other and to the reaction chamber. Preferably, e.g., vertical support structures of the entire reactor plant can be disposed in these spaces. These free spaces can preferably also be used for the disposing of various metering and inspection ports as well as sampling connections and feed ducts for various materials.